1. Technical Field
This invention relates generally to pistons for internal combustion engines, and methods of manufacturing the pistons.
2. Related Art
Engine manufacturers are encountering increasing demands to improve engine efficiencies and performance, including, but not limited to, improving fuel economy, reducing oil consumption, improving fuel systems, increasing compression loads and operating temperatures within the cylinder bores, reducing heat loss through the piston, improving lubrication of component parts, decreasing engine weight and making engines more compact, while at the same time decreasing the costs associated with manufacture.
While desirable to increase the compression load and operation temperature within the combustion chamber, it remains necessary to maintain the temperature of the piston within workable limits. Also, achieving an increase in the compression load and operation temperature comes with a tradeoff in that these desirable “increases” limit the degree to which the piston compression height, and thus, overall piston size and mass can be decreased. This is particularly troublesome with typical piston constructions having a closed or partially closed cooling gallery to reduce the operating temperature of the piston. The cost to manufacture pistons having upper and lower parts joined together along a bond joint to form the closed or partially closed cooling gallery is generally increased due to the joining process used to bond the upper and lower parts together. Further, the degree to which the engine weight can be reduced is impacted by the need to make the aforementioned “cooling gallery-containing” pistons from steel so they can withstand the increase in mechanical and thermal loads imposed on the piston.
Recently, single piece steel pistons without a cooling gallery have been developed and can be referred to as “galleryless” pistons. Such pistons provide for reduced weight, reduced manufacturing costs, and reduced compression height. The galleryless pistons are either spray cooled by a cooling oil nozzle, lightly sprayed for lubrication only, or are not sprayed with any oil. Due to the absence of the cooling gallery, such pistons typically experience higher temperatures than pistons with a conventional cooling gallery. High temperatures can cause oxidation or overheating of an upper combustion surface of the steel piston, which can then cause successive piston cracking and engine failures. High temperatures can also cause oil degradation along an undercrown area of the piston, for example underneath a combustion bowl where the cooling or lubrication oil is sprayed. Another potential problem arising due to high temperatures is that the cooling oil can create a thick layer of carbon in the area where the cooling or lubrication oil is in contact with the piston undercrown. This carbon layer can cause overheating of the piston with potential cracking and engine failure.